A study of cancer risk in approximately 146,000 x-ray technologists who were certified by the American Registry of Radiologic Technologists between 1926-80 is ongoing. The X-ray technologist cohort provides the potential to study exposure to small radiation doses over many years, with some technologists receiving cumulative doses up to as much as 0.2 Gy. This unique occupational cohort is 73% female, offering a rare opportunity to study low-dose effects on the breast and thyroid, two sensitive organ sites for radiation carcinogenesis in women. When the second survey of technologists is completed, this large cohort will become a valuable Division-wide resource for studying a variety of cancers. To develop estimates of occupational radiation exposure for individual x-ray technologist cohort members, collaboration with medical physicists and industrial hygienists from the National Institute for Occupational Safety and Health has recently been formalized. Additionally, an initiative was undertaken to collect blood samples for future genetic studies. Initial efforts are focusing on women with breast plus other (multiple) primary cancers and men with testicular cancer, early-onset (<55) prostate cancer, and prostate plus other primary cancers, but will likely expand to other cancer sites. This effort will include testing for genes that may be involved in DNA repair or repair of radiation damage, such as ATM and RAD51.A cohort of 20,000 Mayak nuclear facility workers (in the former Soviet Union)is being studied because they comprise a unique occupational group for protracted exposure to external radiation at high doses, and exposure to plutonium and strontium. Mortality analyses of the Mayak workers demonstrate that internal exposure to plutonium results in excess risk of bone and liver cancer, with higher risks for females than males. Dosimetry reconstruction is underway to evaluate the shape of the dose-response function. In addition, initial analyses evaluating the effects of external radiation exposure, showed dose-response relationships for leukemia and all solid cancers. Several studies of Chornobyl clean-up workers are currently underway. Cancer incidence is being evaluated in three cohorts, comprising 17,000 clean-up workers, from Estonia, Latvia, and Lithuania who were sent to Chornobyl (Ukraine) following the reactor accident in 1986. Cohort members are interviewed to elicit other risk factor information. In addition, with support from DOE, NRC, and the French Institut de Protection et Surete Nucleaire (IPSN), NCI is in the final phase of a pilot study of leukemia and other hematologic disease among the Chornobyl clean-up workers in Ukraine.